Several data sources can be used to describe fertility behaviour. Data on births recorded in the civil registers can be linked with population data from censuses to provide annual estimates of fertility. Additionally, censuses in Belgium and Family surveys in France contain retrospective questions about the number of children already born to the respondents. Looking at cohorts of women born in the first half of the twentieth century, Sandra Brée, Thierry Eggerickx, Jean-Paul Sanderson and Rafael Costa compare the data from civil registration with the retrospective data obtained via these questions at the national and sub-national levels in France and Belgium. The authors find strong consistency between the data, with only a few exceptions. Combining these different data sources thus provides opportunities to study the fertility of cohorts of women over long periods.